Brittle sedimentary strata focus a multimodal depth distribution of seismicity during hydraulic fracturing in the Sichuan Basin, Southwest China

Mengke An, Derek Elsworth, Fengshou Zhang, Rui Huang, Junlun Li, Zhengyu Xu, Zhen Zhong, Manchao He

Research output: Contribution to journalArticlepeer-review

Abstract

The number of background earthquakes (ML ≥ 0) in the southern Sichuan basin, southwest China, has increased thirtyfold as a result of hydraulic fracturing. Background events are originally deep (4–6 km) within the sedimentary section but build into a multimodal distribution both at depth and in the shallow stimulated reservoir (2–4 km) - representing a counterpoint to the usual triggering of seismicity on deep sub-reservoir basement faults. Surprisingly, the largest events (ML ≥ 3) evolve in the deep sedimentary strata (4–6 km) that are hydraulically isolated from the injection zone (2–4 km) by low permeability layers. We evaluate the friction-stability rheology of the strata within the full stratigraphic section to define the feasibility of nucleation within these shallow and deep strata. These show velocity-neutral to velocity-weakening behavior in the shallow reservoir transitioning to more strongly velocity-weakening with increase in both depth and temperature. Poroelastic stress calculations confirms that stress transfer, rather than transmitted fluid pressures, are capable of directly reactivating critically-stressed faults at depth, with fluid pressures the triggering source within the shallow reservoir.

Original languageEnglish (US)
Article number230332
JournalTectonophysics
Volume880
DOIs
StatePublished - May 10 2024

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Earth-Surface Processes

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